In an air vehicle or aircraft with two coaxially mounted contrarotating or counter rotating rotors, the rotors provide both the lift controlling vertical movement and horizontal velocity. Horizontal flight is achieved by tilting rotors forward at a certain angle. The direction flight or yaw attitude can be controlled in a number of different ways.
With coaxial counter rotating rotors, power is normally evenly distributed between the rotors. The equal distribution of the power causes the torque from each rotor to be canceled by the other rotor such that no net torque reaction is passed to the vehicle body. With the torque balanced, the air vehicle will remain in the desired direction. To change the direction of the air vehicle a propeller can be vertically mounted on a tail or other horizontal extension of the air vehicle. Yaw control can then be provided by controlling this propeller, for example, by the angle of the blades of that vertical propeller.
With the two counter rotating rotors, yaw control can also be obtained by unbalancing the power distribution between the rotors, for example, by varying the pitch of the blades of one rotor relative to the blades of the other rotor. However, this yaw control system requires expensive and complicated mechanical mechanisms and is difficult to operate.
Other yaw control systems for air vehicles with contrarotating rotors include a yaw control motor which applies a variable torque to the gears of the rotor transmission, as disclosed in U.S. Pat. No. 3,669,564 to Garfinkle. Another arrangement providing yaw control through the drive gear mechanism for the counter rotating rotors is disclosed in U.S. Pat. No. 3,735,945 to Huvers, which discloses the use of a variable clutch located between idler gears of the drive trains of the two rotors. The Huvers clutch provides varying torques to the idler gears, and thereby controls the resulting torque applied to the air vehicle body to turn the air vehicle in either direction for yaw control. These two systems for providing yaw control through the differential gear reduction mechanism are difficult and costly to manufacture and operate.
An external yaw control system is disclosed in U.S. Pat. No. 3,554,467 to Yowell which provides exteriorly mounted vertical stabilizers which are pivoted to the air frame within the down draft of the counter rotating rotors. Yaw control is provided by controlling the pivoted angle of the vertical stabilizers. However, these external vertical stabilizers, like the external vertical propeller, further complicate and increase the size of the air vehicle.